1. Field of the Invention
The present invention relates to a foreign particle inspection apparatus for detecting foreign particles adhered on circuit patterns such as reticles or photomasks (hereinafter called reticles, etc.). More specifically, the invention relates to a foreign particle inspection apparatus (the inspection is performed before the circuit patterns are transferred to a wafer) for detecting small foreign particles of the order of submicrons on the above reticles, particularly on reticles having a phase shift film for improving the patterning resolution by a simple structure.
2. Description of the Prior Art
In the exposure process of reticles, etc. used for manufacturing LSIs or PCBs, each circuit pattern such as a reticle is inspected before it is baked and transferred onto a wafer. However, there is a problem imposed that even when small foreign particles, for example, of the order of microns are adhered on the above circuit pattern, the above circuit pattern is not correctly transferred onto a wafer due to the above foreign particles, and hence all of the LSI chips become defective. This problem is getting more remarkable due to recent high integration of LSIs and the existence of smaller foreign particles of the order of submicrons is not allowable.
To prevent the above defective transfer, it is essential to inspect foreign particles prior to the exposure process. Various foreign particle inspection techniques have been proposed for controlling reticles. For inspection of a circuit pattern such as a reticle, a method that a laser beam with superior directivity is irradiated slantwise onto it and scattered light generated from foreign particles is detected is advantageous from a view point of inspection speed and sensitivity and generally used.
In the above inspection method, however, diffracted light is also generated from the edge of the circuit pattern such as a reticle and a means for discriminating and detecting only foreign particles from the diffracted light is necessary and the techniques for that purpose are disclosed as shown below.
The first one of them is a foreign particle inspection apparatus comprising a linearly polarized laser, a means for irradiating the above laser beam slantwise at a specific incident angle, and a slant imaging optical system using a polarizing plate and lenses as disclosed, for example, in Japanese Patent Laid-Open No. 54-101390 (U.S. Pat. No. 4,342,515). The apparatus detects only foreign particles which are made bright by using the character that when linearly polarized light is irradiated onto a circuit pattern, the diffracted light from the circuit pattern and the scattered light from the foreign particles are different in the light polarizing direction from each other.
The second one of them is a foreign particle inspection apparatus comprising a means for irradiating and scanning a laser beam slantwise onto an inspection sample, a first lens which is installed above the inspection sample so as to allow the irradiation point of the above laser beam to almost coincide with its local point plane and condenses scattered light of the above laser beam, a shutter which is installed on the Fourier transform plane of the above first lens and shields regularly diffracted light from the circuit pattern of the inspection sample, a second lens for subjecting scattered light from foreign particles which is obtained via the shutter to an inverse Fourier transform, a slit which is installed at the imaging point of the above second lens and shields scattered light from other than the laser beam irradiation point on the inspection sample, and a light receptor for receiving the scattered light from the foreign particles which passes through the above slit, which is disclosed in, for example, Japanese Patent Laid-Open No. 59-65428, Japanese Patent Laid-Open No. 01-117024 (U.S. Pat. No. 5,046,847) and, Japanese Patent Laid-Open No. 01-153943.
In consideration of that the circuit pattern is generally structured in the same direction or by a combination of two or three directions in the view field, the above apparatus eliminates the diffracted light by the circuit pattern in this direction by a spatial filter installed on the Fourier transform plane so as to emphasize and detect only the scattered light from the foreign particles.
The third one of them is a foreign particle inspection apparatus of a structure that foreign particles are discriminated by ANDing detection outputs of a plurality of detectors which are installed slantwise in consideration of that diffracted light generated at the circuit pattern edge is directive but scattered light by foreign particles is not directive as disclosed, for example, in Japanese Patent Laid-Open No. 57-80546.
The fourth one of them is a foreign particle inspection apparatus for discriminating foreign particles by arranging a plurality of detectors using a phenomenon that diffracted light from the circuit pattern edge is focused only in a predetermined direction, while light generated by the foreign particles is scattered in all directions as disclosed, for example, in Japanese Patent Laid-Open No. 60-154634 and Japanese Patent Laid-Open No. 60- 54635. When an array detector such as a one-dimensional solid imaging device, foreign particles are detected among pixels constituting the array and the output from the foreign particles is dispersed and detected into a plurality of pixels. As a result, the output from the detector becomes small in correspondence with dispersion and there is a possibility to miss foreign particles.
The fifth one of them is a foreign particle inspection apparatus using a method that array detectors are installed with an inclination to the scanning direction of the sample stage as disclosed in Japanese Patent Laid-Open No. 61-104242 or using a method that specially shaped array detectors are specially arranged as disclosed in Japanese Patent Laid-Open No. 61-104659 and, so as to avoid the above missing of foreign particles. Uneven or variable illumination affects the revivability and accuracy of detection.
The sixth one of them is a foreign particle inspection apparatus providing automatic calibration using a standard sample wherein the intensity of scattered light is measured beforehand as disclosed, for example, in Japanese Patent Laid-Open No. 60-38827 and,
The seventh one of them is a foreign particle inspection apparatus using a means for preventing misidentifying a large amount of scattered light generated from a large foreign particle as scattered light from a number of small foreign particles as disclosed in Japanese Patent Laid-Open No. 1981-132549.
Techniques on the Schrielen method, a phase contrast microscopy, and diffraction images of a finite light source as a method and apparatus relating to small foreign particle inspection are disclosed, for example, in H. Kubota, Applied Optics (Iwanami Zensho), pp. 129 to 136.
As mentioned above, as foreign particles to be detected become small, an increase in missing of foreign particles affecting the manufacture of LSIs comes into question.
In the first one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 54-101390 and, since the difference between the polarizing direction of scattered light from small foreign particles and the polarizing direction of diffracted light from the circuit pattern edge is small, a problem such that the foreign particles cannot be discriminated and detected is imposed.
Next, in the second one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 59-65428, Japanese Patent Laid-Open No. 01-117024, and Japanese Patent Laid-Open No. 1989-153943), scattered light from foreign particles is separated from diffracted light from the circuit pattern by the shutter and only the scattered light from the foreign particles is detected by the slit. This prior art has a characteristic that the detection mechanism is simplified because foreign particles are detected by a simple binarizing method. However, diffracted light from the transposition parts of the above circuit pattern has a low tendency to be one-sided at a specific location like diffracted light from the straight lines and the diffracted light from the transposition parts of the circuit pattern cannot be shielded perfectly by the above spatial filter.
Furthermore, since diffracted light generated from a circuit pattern having a fine structure pattern of the order of microns required by recent high integration of LSIs is similar to scattered light from foreign particles in behavior, the above tendency is growing moreover and it is practically difficult to separate and detect foreign particles from the circuit pattern by the simple binarizing method, causing a problem.
In each apparatus of the third one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 57-80546) and the fourth one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 60-154634 and Japanese Patent Laid-Open No. 60-154635 and, there are problems imposed that it is difficult to use an optical system having a sufficient condensing capability from a view point of the equipment structure and it is practically difficult to detect weak scattered light generated from small foreign particles.
In each apparatus of the fifth one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 61-104244 and Japanese Patent Laid-Open No. 61-104242), there are problems imposed that it is necessary to manufacture special detectors arid use a special optical system from a view point of the equipment structure and the application cost goes up.
Furthermore, in the apparatus of the sixth one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 60-038827 and,), there are difficulties in the structural accuracy for array detectors suited to high speed detection and for detection of small foreign particles.
Furthermore, in the apparatus of the seventh one of the prior arts mentioned above (for example, Japanese Patent Laid-Open No. 56-132549), there is a problem imposed that since only one point of a large foreign particle is assumed as a foreign particle, the shape of a particularly long and narrow foreign particle cannot be recognized correctly.
To improve the patterning resolution of circuit patterns on a reticle formed by a metallic thin film such as chromium, a reticle whereon transparent or opaque thin films (the film thickness is about odd number times of a half of the wave length of the exposure light source) which are called phase shift films or phase shifters are mounted between the circuit patterns thereon has been developed recently. Although this film is transparent or opaque, it has a structure which is several times as large as the circuit pattern (about 0.1 .mu.m in thickness). Therefore, diffracted light from the film edge is several to several tens times as large as diffracted light from the conventional circuit pattern edge and the foreign particle detection sensitivity is extremely lowered. Various trials have been made so as to solve those problems (for example, Japanese Patent Laid-Open No. 63-315936 application No. 62-151121 (U.S. Pat. No. 4,952,058), though no satisfactory solutions to those problems are found yet.
Therefore, it is practically difficult to separate and detect foreign particles from the circuit pattern by a simple linear spatial filter, causing a problem.